Anything is possible when you book a guarantee category. Perhaps you are in a category that is just 3 or 4 decks below your favorite pool or the buffet which might be at the front or back of the ship.

Most lines typically rate the best rooms to be mid-ship. Perhaps they might move you to mid-ship which they classify as the best category but you might not like it. Maybe its near an elevator or just has too much hallway traffic.

Bottom line is, if you want a specific location and/or room, book an actual room and tell your agent to mark in your booking you don't want an upgrade.

Yes, it's very possible to be "upgraded" to a cabin you might not like as much, such as one under the Lido deck, a restaurant or a lounge, or other noisy location.

If you already have your cabin assignment it's very unusual for that to change, but it could happen. If you're very happy with the cabin you've been assigned you can contact whomever you booked with and ask them to mark it "no upgrade". Of course, you might miss a great upgrade .